Method of surface treatment for dovetail in gas turbine engine fan blade

ABSTRACT

A method of surface treating a dovetail in a blade includes the steps of providing a burnishing operation along circumferential sides of the dovetail. Next, shot peening is provided at axial ends of the dovetail. A fan blade made by the method is also claimed.

BACKGROUND

This application relates to a surface treatment method for treating adovetail in a blade.

Gas turbine engines are known, and typically have a fan delivering airto a compressor. From the compressor, the air passes downstream into acombustion section where it is mixed with fuel and burned. Products ofthat combustion pass downstream over turbine rotors which in turn drivethe fan to rotate.

The fan may include a plurality of separate blades mounted into a rotor.The blades have an airfoil which moves the air, and a radially innerportion, known as a dovetail, which is received within a slot in therotor.

The dovetail is subject to high stress during operation of the gasturbine engine. Thus, it presents challenges to a designer of a fanblade.

One method that has been proposed to address the stresses applied intothe dovetail is to utilize a burnishing operation on the dovetail. Aburnishing operation will induce stresses within the part. These inducedstresses are residual compressive stresses that can be induced into thepart to a desired depth. The induced stresses counter applied stressesduring operation to result in an overall lower stress level.

The burnishing operation does provide good control over the level ofinduced stresses and the depth into the material. However, burnishingtypically requires a somewhat flat surface, and thus, complex surfacesmay not be candidates for burnishing.

Once a burnishing operation has been applied to a part, surfaces thathave not been burnished may have increased stresses or other changeswhich may be undesirable.

SUMMARY

A method of surface treating a dovetail in a blade includes the steps ofproviding a burnishing operation along circumferential sides of thedovetail. Next, shot peening is provided at axial ends of the dovetail.A blade made by the method is also claimed.

These and other features of this application will be better understoodfrom the following specification and drawings, the following of which isa brief description:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a fan blade.

FIG. 1B shows the fan blade mounted into a rotor.

FIG. 2 shows a first method step according to this invention.

FIG. 3 shows a subsequent step.

DETAILED DESCRIPTION

FIG. 1A shows a fan blade 20 having an airfoil 18 extending radiallyoutwardly of a dovetail 24. The airfoil can be said to have a leadingedge 21 and a trailing edge 22. The dovetail 24 has circumferential sidesurfaces 100, and axial ends 102 and 104. As can be appreciated fromFIG. 1A, the axial ends 102 and 104 are not regular surfaces, but havecomplex curved shapes. The circumferential sides 100 have generallyregular shapes along an axial length.

FIG. 1B shows the fan blade 20 mounted within a slot 17 in a rotor 16.As can be appreciated, the sides 100 receive a good deal of stress whenthe rotor 16 is rotating, as the rotational force is transmitted throughthe sides 100. The ends 102 and 104 do not receive the same forces. Asshown, the circumferential sides 100 have a more regular surface thanthe ends 102 and 104.

In order to surface treat the fan blade dovetail to withstand suchforces, a first surface treatment step, burnishing, is shown in FIG. 2.The term burnishing can be generally described as bringing a tool intocontact to physically treat surfaces on a workpiece.

A burnishing operation with a burnishing tool, shown ratherschematically at 50, has surfaces 104 brought into contact with the sidesurfaces 100. The burnishing tool 50 may provide low plasticityburnishing. One known low plasticity technique is disclosed in U.S. Pat.No. 6,415,486. Other burnishing operations can be used however.

As shown in FIG. 2, in a generic burnishing operation, the tool 50 iscontrolled with the computer numerical control (CNC) positioning control56, and a pressure control 58. Broadly speaking, once the selected area100 of the dovetail has been identified, a desired magnitude ofcompression from the tool surfaces 52 and a residual desired stressdistribution are determined. The pressure is controlled against thesurfaces to form zones of deformation having a deep layer of compressivestress. The pressure can be varied to achieve a desired residual stressdistribution, and a magnitude of compression within the dovetail 24.

While burnishing works very well on the circumferential side surfaces100, it is not as effective for the ends 102 and 104. Since the ends 102and 104 have complex surfaces, they are not as easily treated with theburnishing technique. In addition, the burnishing of the sides 100 doesresult in undesirable stress distribution at the ends 102 and 104.

Thus, FIG. 3 shows a step subsequent to the FIG. 2 step. In FIG. 3, shotpeening masks 62 are mounted to enclose the ends 102 and 104. Shotpeening tool 60 delivers particles 64 against the ends 102 and 104. Theshot peening process is shown somewhat schematically in FIG. 3, however,a worker of ordinary skill in the art would recognize how to provideshot peening to the surfaces 102 and 104.

After the two-step method is complete, the fan blade 20 has been surfacetreated at both of its ends 102/104, and the sides 100 of the dovetailsuch that it will provide effective operation in the challengingenvironment of the gas turbine engine.

In addition, as an initial optional step prior to the FIG. 2 step, theentire dovetail may be initially shot peened prior to the burnishing.

The blade as disclosed above is for use as a fan blade. It is believedthe teachings of this application would have most success when utilizedon blades that will exist in lower temperature portions of a gas turbineengine. However, it is possible that blades in higher temperatureregions may also benefit from the methods. In addition, the methods mayhave application in blades having air foils and dovetails for use inapplications other than gas turbine engines. Thus, while the inventionmight have most application in fan blades and lower pressure compressorsections, it may also have application in other locations.

Although an embodiment has been disclosed, a worker of ordinary skill inthe art would recognize that certain modification would come within thescope of this invention. For that reason, the following claims should bestudied to determine the true scope and content of this invention.

What is claimed is:
 1. A method of surface treating a dovetail in ablade comprises the steps of: providing a burnishing operation alongcircumferential sides of said dovetail; and then providing shot peeningto axial ends of said dovetail.
 2. The method as set forth in claim 1,wherein said burnishing operation is low plasticity burnishing.
 3. Themethod as set forth in claim 2, wherein said burnishing techniqueincludes a body which is brought into contact with the circumferentialsides of said dovetail, said body being controlled by a position controland a pressure control to apply a desired stress distribution andcompressive load within the dovetail along said circumferential sides.4. The method as set forth in claim 1, wherein the entire dovetail issubject to a shot peening process prior to the burnishing operation. 5.The method as set forth in claim 1, wherein said axial ends are notregular surfaces, but have complex curved shapes
 6. The method as setforth in claim 5, wherein said circumferential sides have a more regularshape.
 7. The method as set forth in claim 1, wherein said blade is foruse in a gas turbine engine.
 8. The method as set forth in claim 7,wherein said blade is a fan blade.
 9. A blade comprising: an airfoilextending radially outwardly from a dovetail, the airfoil having aleading edge and a trailing edge; and said dovetail havingcircumferential sides, and axial ends, with said circumferential sidesbeing surface treated by a burnishing operation, and said axial endsbeing surface treated by shop peening; and said axial ends havingcomplex curved shapes, and said circumferential sides having generallyregular shapes along an axial length.
 10. The blade as set forth inclaim 9, wherein said blade is for use in a gas turbine engine.
 11. Theblade as set forth in claim 10, wherein said blade is a fan blade.